[meteorite-list] First Triple Asteroid System Found

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Aug 10 17:04:47 2005
Message-ID: <200508102103.j7AL3ob10583_at_zagami.jpl.nasa.gov>

http://www.berkeley.edu/news/media/releases/2005/08/10_sylvia.shtml

UC Berkeley Press Release

First triple asteroid system found
By Robert Sanders
10 August 2005

BERKELEY - One of the thousands of asteroids orbiting the sun has been
found to have a mini planetary system of its own.

University of California, Berkeley, assistant research astronomer Franck
Marchis and his colleagues at the Observatoire de Paris have discovered
the first triple asteroid system - two small asteroids orbiting a larger
one known since 1866 as 87 Sylvia.

Because 87 Sylvia was named after Rhea Sylvia, the mythical mother of
the founders of Rome, Marchis proposed naming the twin moons after those
founders: Romulus and Remus. The International Astronomical Union (IAU)
approved the names, to be announced in its Aug. 11 circular.

Marchis and his colleagues will report their discovery in the Aug. 11
issue of the journal Nature simultaneously with an announcement that day
at the Asteroid Comet Meteor conference in Arma??o dos B?zios, in the
Brazilian state of Rio de Janeiro.

The asteroid 87 Sylvia is one of the largest known from the asteroid
main belt, which is located between the orbits of Mars and Jupiter.
Shaped like a lumpy potato, Sylvia is about 280 kilometers (175 miles)
in diameter and is located in the Cybele outer part of the belt, about
3.5 astronomical units (AU) from the sun. An AU is 93 million miles, the
average distance between the sun and Earth.

Artist's conception of Sylvia 87 and its moons
Artist's conception shows twin moonlets, Romulus and Remus, orbiting the
large main-belt asteroid 87 Sylvia. (Image courtesy European Southern
Observatory)

Watch video
Animated drawing of asteroids
<http://astron.berkeley.edu/%7Efmarchis/document/Sylvia/Movies/ESO_movie_SMALL.mov>
Animated artist's rendering of asteroids
<http://astron.berkeley.edu/%7Efmarchis/document/Sylvia/Movies/ESO_movie_SMALL.mov>
(8 Mb .MOV file)
Animation of apparent orbits of Romulus and Remus
<http://astron.berkeley.edu/%7Efmarchis/document/Sylvia/Movies/sylvia_animAug2004.mpg>
Animation of apparent orbits of Romulus and Remus
<http://astron.berkeley.edu/%7Efmarchis/document/Sylvia/Movies/sylvia_animAug2004.mpg>
(2.9 Mb .MPG file)

Four years ago, Sylvia was discovered to have a moon, making it one of
some 60 known binary asteroids in various asteroid populations of the
solar system. Seventeen of these binary systems are in the main asteroid
belt and have been imaged directly either by adaptive optics systems on
large, ground-based telescopes or by the Hubble Space Telescope.

Now, a second moon has been seen around Sylvia, making it a triple
asteroid system. Sylvia's newly discovered moons orbit in nearly
circular orbits in the same plane and direction (prograde) as the moon
orbits the Earth. The closest moonlet, orbiting about 710 km (450 miles)
from Sylvia, is Remus, a body only 7 km (4.4 miles) across and circling
Sylvia every 33 hours. The second, Romulus, orbits at about 1360 km (860
miles), measures about 18 km (11.3 miles) across, and orbits in 87.6
hours. The asteroid Sylvia spins at a rapid rate, once every 5 hours and
11 minutes.

"People have been looking for multiple asteroid systems for a long time,
because binary asteroid systems in the main belt seem to be common and
formation scenarios, such as a collision between two asteroids followed
by disruption and re-accretion, suggest that fragments should be
orbiting bigger asteroids," Marchis said. "I couldn't believe we found
one."

>From two months' of observations of the moonlets' orbits, Marchis and
his Paris colleagues were able to precisely calculate the mass and
density of Sylvia, which shows it to be a "rubble-pile" asteroid,
Marchis said. These asteroids are loose aggregations of rock presumably
created when one asteroid smacked into another, disrupting one or both
of them. A new asteroid formed later by accretion of large fragments
from the disruption. The moonlets probably are debris left over from the
collision that were gravitationally captured by the newly formed
asteroid and which eventually settled into orbits around it.

"That's why most main-belt asteroids with companions have a rubble-pile
structure," he said. "Because of the scenarios of their formation, we
expect to see more multiple asteroid systems like this."

The density, 1.2 grams per cubic centimeter, is 20 percent higher than
the density of water, which suggests it is composed of water, ice and
rubble from a primordial asteroid, probably a hydrated carbonaceous
chondrite, based on previous spectroscopic studies of the asteroid.

"It could be up to 60 percent empty space," said astronomer Daniel
Hestroffer, one of three coauthors from the Institut de M?canique
C?leste et Calculs d'?ph?m?rides at the Observatoire de Paris.

The discovery was made with one of the European Southern Observatory's
8-meter telescopes (Yepun) of the Very Large Telescope at Cerro Paranal,
using the telescope's infrared camera and the high angular resolution
provided by the adaptive optics system (NACO). Via the observatory's
promising "service observing mode," Marchis and his colleagues were able
to obtain sky images of many asteroids over a six-month period without
actually having to travel to Chile. DVD data of the observations were
sent regularly via mail to Berkeley.

Marchis had the discovery sitting on his shelf for months, since
November 2004, because he waited for the completion of the project
before starting to process the data and before sending them to colleague
Pascal Descamps of the Observatoire de Paris. Just as Marchis was set to
go on vacation in March 2005, Descamps sent him a brief note entitled
"87 Sylvia est triple?" pointing out that he could see two moonlets
around several images of Sylvia. The entire team then focused quickly on
analysis of the data, wrote a paper, submitted an abstract to the August
meeting in Rio de Janeiro and submitted a naming proposal to the IAU.

Marchis and his colleagues hope to use the adaptive optics of the Keck
and the Gemini telescopes to obtain better images of the triple-asteroid
system in order to pin down the precise orbits, verify Sylvia's
formation scenario and chart the system's evolution. Already they see
precession of the moon's orbits resulting from the irregular shape of
Sylvia.

The work was partially supported by the National Science Foundation and
the Technology Center for Adaptive Optics and by the Chretien
International Research Grant of the American Astronomical Society.

The fourth author with Marchis, Descamps and Hestroffer was J?r?me
Berthier, also of the Institut de M?canique C?leste et Calculs
d'?ph?m?rides at the Observatoire de Paris. The moon now designated
Romulus was discovered in 2001 by M. E. Brown and J. L. Margot using the
Keck II telescope atop Mauna Kea in Hawaii.

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[Image}
Sylvia the wolf suckling Romulus and Remus
Statue of the she-wolf Sylvia sucking the twins Romulus and Remus.
(Courtesy UC Santa Cruz Library / Branson de Cou Archive)

The mythical Romulus and Remus

As related by the Roman historian Livy, who lived around the time of
Christ, Rhea Silvia was forced to become a vestal virgin by her uncle,
Amulius, after he overthrew her father, Numitor, king of the ancient
Italian city of Alba Longa. Amulius wanted to ensure that Sylvia would
not have any sons who could attempt to overthrow him. Despite her virgin
priestess status, Mars, the god of war, came to Sylvia in her temple and
conceived with her twin sons, Romulus and Remus. When they were born,
Amulius ordered a servant to kill the twins, but a merciful servant set
them adrift in the river Tiber.

The twin boys were found by Tiberinus, the river god, and nursed by a
female wolf underneath a fig tree. Romulus and Remus were eventually
discovered by Faustulus, a shepherd, who brought the children to his
home and, with his wife, Acca Larentia, raised the boys as their own.
Upon reaching adulthood, Romulus and Remus returned and killed Amulius
and reinstated Numitor, their grandfather, as king of Alba Longa.

The two subsequently built a settlement on the Palatine Hill, where they
had been nursed by the she-wolf, beginning on April 21, 753 BC, the
traditional date for the founding of Rome. During the building, Remus
thoughtlessly jumped the unfinished city wall, an omen of ill fortune,
and Romulus killed him. Romulus named the city Roma, after himself, and
made himself king.

Marchis proposed Romulus as the name of the largest moonlet because the
name means "forceful" (rhome in Latin). The name Remus is related to the
word "cautious" (remorari) and seemed more appropriate to the smallest
moon and the closest to Sylvia.
Received on Wed 10 Aug 2005 05:03:50 PM PDT